Molten salt spray process for descaling stainless steel



J. A. FALER' March 24, 1964 2 Sheets-Sheet 1 Filed Sept. ll, 1961 4 N fi W N/// z N E mm m & W

ATTORNEYS J. A. FALER March 24, 1964 MOLTEN SALT SPRY PROCESS FOR DESCALING STAINLES S STEEL Filed Sept. ll, 1961 2 Sheets-Sheet 2 JNVENTk.

JOHN A. FALER BY ATTORNEYS United States Patent O 3,126,3o1 MOLTEN SALT SPRAY PROCESS FOR DESCALING STAINLESS STEEL John A. Falet', Detroit, Mich., assigor to Kolee Corporation, Detroit, Mich. Filed Sept. `'11, 1961, Ser. No. 137,319 4 Claims. (Cl. 134-29) stainless steel has been cleaned successfully in immersion baths. See Dunlevy 2,635,062 of April 14, 1953, and Webster 2, 458,66l of January ll, 1949. The cleaning operation removes oxides and gives an excellent finish to the material, bright, high bufiing quality, and high corrosion resistance.

This operation is more than a cleaning operation; it is a process which conditions the stainless steel strip by oxidizing the scale, after which the thus oxidized scale may then be processed in accordance with the process of Patent No. 2,458,661 or any other process as, for example, through acid pickling, etc.

However, it was found that for various reasons, scratching and marring of the strip occurred, explained because of the presence of insoluble particles on the strip and also because of the transverse movements of the strip on the metal rolls. Rolls are needed to guide the strip through the bath, and metal has to be used for the rolls.

In addition to scratching and marring of the strip because of particles on the strip and transverse movements of the strip under metal guide rolls, or over metal guide rolls, another important fact to consider is that the more rolls used for starting strip under or over them, the greater the amount of tension. This results in an actual elongation of strip under some conditions, or a certain amount of distortion, sometimes causing so-called cross breaks which run across the surface of the strip, or irregularities lengthwise, such as depressions or raised surfaces due to this tension. It is desirable to eliminate rolls that are used to guide strip and, theoretically, so the pass line could be straight-with no dipping into any tanks, whether it be salt water, acid or other solutions` As a result, the immersion bath process is limited, by commercial factors, to intermediate grades of stainless steel. When it came to cleaning of very high quality stainless steel, the immersion bath process has not been widely adopted and instead, processors for these higher grades of stainless steel continue to resort to other and older methods of cleaning, such as acid treatment.

The immersion process also was not widely adopted for very high quality stainless steel, nor extremely light gauges of stainless steel (such as below .O"), particularly finished product Where on additonal cold rolling is used.

Hence, there has been developed and here disclosed a new process for cleaning stainless steel, and this process, is the subject matter of this application. In the process, the stainless steel in strip form comes out of an annealing oven and into a spray oven or mufile or box where it is cleaned or oxidized by molten salt spray and then emerges into the customary treatments such as water rinsing, quenchng, dilute acid picklng, rinsing, and then rewinding. The novelty here is the spray oven and what happens in the spray oven where molten salt spray is thrown onto the strip as it leaves the annealing furnace.

The spray muflle or oven under consideration comprises an enclosed spray zone which in practical structural form may be considered as a box into which the hot strip enters at one end from the annealing furnace and-from which the oxdized or cleaned strip emerges at the other end, thence passing into a rinsing zone which may be a rinsing bath or water spray.

The oxidizing oven or spray box, as we may call it,

&126301 Paterted Mar. 24, 1964 ice receives the hot steel strip from the annealing furnace and discharges it to the rinsirg bath. In the oxidizing oven, there are no rolls to be engaged by the moving strip but the strip is moved freely under tension through the oven, entering a slot in one end and leaving through a slot in the other end and at no time in the oxidizing oven is the strip in contact with any rolls or supports'or guides;

The strip is not in contactwith metal rolls as it passes through the spray box. Hence there is no electrolytic or no galvanic action, as would be encountered in an immersion bath containing salt and metal rolls over which a metal strip passes.

The hot steel strip enters at a temperature of about 1000 F. and emerges at about the same temperature, there being no substantial drop in temperature of the strip until it enters the water rinse and quench. It is important to maintain the strip at substantially the same temperature all the Way through the oxidixing oven. As long as the strip is at high temperature, in the presence of a molten salt spray, the oxidizing action will take place without the formation of any solid particles that might otherwise abrade or scratch the strip. The strip enters hot and is maintained at substantially the same tempera ture all the way through the oxidzing oven.

The oxidizing action is obtained by subjecting the strip and the atmosphere within the oven to a molten salt spray; such as Kolene #1, see Patent No. 2,458,661, for example, and the spray is formed by atomizing the molten salt, using large quantities of an inert gas such assuperheated steam at about 1000 F., which passes into nozzles, into which is fed in very tiny quantities and in small increments the molten salts which, at that point, is at a temperature of 1000 F. The exact temperature is not critical, except that it exceed the melting point of the salt spray, about 550 F., a substantial range being permitted.

The interior of the oven is also maintained charged with an atmosphere of super-heated steam coming from different outlets from the steam super-heater. In other words, some of the steam goes out through steam nozzles into the atmosphere for heating the atmosphere and filling it full of steam, and another part of the steam goes through the atomizing nozzles.

The steam mainta'ns a hot steam atmosphere in the oven so that in the event of a possibility of some of the salt coming through in unatomized condition from the nozzles, then this salt will be carried around by the steam and be maintained molten by the steam so that at no point can any solidified salt particle engage the strip as it goes through the oven.

Another important reason for loading the oven atmosphere with steam is to prevent the possibility of any gas, such as air, entering the oven. Steam is inert to the salt. Other atmospheres, such as air, would react with the salt and form solid particles which would impair the operation. By using steam', or similar inert gas, we insure against anything else coming into the oven, and the steam coming in under super-heated conditions seals the slots through which the strip is passing and prevents air from entering the oven at such slots. Otherwise, the oven is maintained sealed whenever possible, and even where there are breaks in the seal, the fact that the oven is loaded with steam seals these breaks against the entrance of air into the oven from outside.

The pressure of 1000 F. steam is greater than the pressure of the atmosphere outside of the oven. It seems that when atmospheric air comes in contact with a heated steel surface, certain higher forms of oxides are promoted. Since it is a desire and intention, by the use of the molten salt spray process, to form no oxide or a form of oxide more easily soluble, it would appear that, since the pressure of steam does not allow atmosphere to' enter Y the oven, while the strip is hot, the lower forms of oxide formed in the presence of steam with the exclusion of air is more desirable, and this is another attribute of the molten salt spray process.

In addition, to insure the temperature within the oven remaining at the desired 1000 F., and in order to provide a safety factor in the event the steam does not function, additional heating means may be employed to maintain the atmosphere at the desired temperature. Such additional or auxiliary heating means may or may not be used in the actual operation from time to time, but is initially provided as equipment so that it is available when needed. This additional means could comprise combustion burners for discharging large quantites of hot combustion vapors into a jacket around the oven, and such jacket is sealed from the oven so that no combustion vapors can enter the oven, and the oven is maintained free of any atmosphere except inert gas, such as steam.

The auxliary heating means, of course, could be electric heaters or any other type of heater, just so long as it beats from the outside and does not discharge any harmful vapors into the oven. If air or combustion vapors were to enter the oven, harm might result. The air or vapors contain CO and because CO by reaction with the salt may form carbonates (CO and because these carbonates are solidified, there might result a plugging of the nozzles through which the molten salt is atomized and, also a marring of the strip by the solid particles.

The super-heated steam moves the atomized salt spray with very high velocity onto the strip and by so doing, concentrates the scouring and chemical action of the molten salt onto the strip and, thus, reduces possible wastage of molten salt. It is estimated that at least 90% of the molten salt is actually used in the reaction that takes place of the salt on the scale and converts it into ozides, and no more than is unused in the process.

The salt spray also has a scale loosening, scouring, or scrubbing action. Impinging on the strip, it facilitates oxidizing of the scale. V

At the discharge end, and preferably at the lowest point of the discharge end, there is a condenser through which is exhausted steam and excess salt vapors. The condenser removes and exhausts the oven, which is constantly receiving a supply of fresh salt and fresh steam, and at least 90% of the salt enters into the chemical change.

The system operates in such a way that any scale or slndge formed is removed in the form of vapors Suspended in the steam and exhausted through the condenser and the exhaust system.

The dragout losses are extremely small here because instead of having a molten salt film of excess amounts on the strip, as is conventional with bath type processes, such excess molten salt film being needed to lubricate the strip as it passes over the rolls, here we use only as much salt as is absolutely necessary to create the desired reactions and, hence, the dragout loss is reduced to an absolute minimum.

The fact that the salt approaches the strip in atomized condition, and under high pressure, creates an almost instantaneous or flash-type Conversion of the oxide which means that the reaction takes place much qnicker and is believed to result in a greatly improved chemical reation. It is well known that a fast reaction in small increments is often highly superior in its chemical action to a slow, prolonged reaction.

Not only is there a saving in salt because of the reduced wastage due to dragout losses, but also a considerably greater amount of the salt enters into the reaction and gives a consderably greater degree of cleaning action, or oxidizing action, than might otherwise be obtained where a strip passes through a bath. Here, because of the atomized condition of the salt impinging upon the strip under high pressure, there is a highly improved cleaning action as contrasted with the action that takes place in immersion baths.

The small amount of salt that is melted for use in the molten salt spray process hereof is always free of buildups of complex metallic impurities and metallic salts such as chrome oxides, iror oxides, nickel oxides, manganese oxides, titanium oxides and others, as might be found in immersion baths. While, to the best of our knowledge, no complete analysis has been made as to how the buildup of these metallic salts affects descaling in salt baths, we are reasonably sure they do have a deleterious eiect and that they impede the reaction.

The salt may be anhydrous. It might also contain some water; or it can be a concentrated solution which is heated, pressurized and superheated before it is sprayed onto the strip. In such case, fiashing off the water allows the salt to mpirge as usual on the metal in anhydrous form.

Now having described the process hereof, reference is had to the appended drawings showing a typical apparatus.

In these drawings:

FIG. 1 shows diagrammatically one form of such apparatus in side view, with parts cut away for purposes of clarity.

FlG. 2 is a section on line 2-2 of FIG. 1.

The drawing shows an insulated steam box, mufile, or oven 10, supported on legs 11 and having entrance and exit slots 12-14. The metal strip 20 being cleaned enters slot 12 after leaving an uncoiling means not shown and a pre-spray heating or annealing means 16, and leaves slot 14 on its way to rinsing means, acid treatment means, heating means, recoiling means, etc., also not shown. In the straight through pass from slot 12 to slot 14 the strip 20 is processed by the process of this application.

The box 10 is inclined as shown so that products formed or deposited in the box collect at a discharge point 22 from where they are removed by a suitable removal means, such as a condenser not shown, including an exhausting fan.

Solid salt, in the instance here shown is fed into hoppers 24 and passes into melting chambers 26 Where it is melted by steam admitted from outside the box 10 through a steam l-ine 27, a steam manifold 28, and pipe terminals 30. In the steam lines connecting manifold 28 to terminals 30 are siphon nozzles 32 which siphon the molten salt from chambers 26 and spray it onto strip 20 immediately after the strip leaves annealing means 16, as soon thereafter as possible. The timing is such that when the strip reaches the salt spray nozzles, the strip is at a maximum annealed temperature; but by the time it leaves exit slot 14 and passes out of the steam atmosphere and into the room air atmosphere, it has cooled down to a point where it is not seriously affected by room air.

Additional steam nozzles or lines 34 admit steam into the box to spray live steam into the box to form a hot steam atmosphere therein at above atmospheric pressure.

Inside box 10 is a steam chest 36 supplied by lines 37 which ruaintains a live steam heated wall inside the box.

The following example llustrates the practice of this invention:

Example 1 A salt composition comprising 2 parts by weight of caustic soda, 1 part by weight sodium nitrate and 0.3 part by weight of sodium chloride are heated with steam at D F. to fusion, picked up as an atomized liquid, and sprayed in a high pressure blast upon both sides of a stainless 8-18 chromium nickel steel strip as it leaves an annealing oven maintained at 1500 F., the metal strip moving at a rate of about 1 ft. per minute and slowly cooling from the annealing temperature to the steam cleaning temperature. The strip is then sprayed with steam alone to clean off any adhering molten salts. The strip, as it leaves the steam cleaning chamber box 10, as described above, is finally quenched with a spray of steam at a temperature of 250' F. At no point in the continuous passage from the annealing oven through the salt spray and quench is the steel strip contacted with rolls. It is finally dipped in a dilute 5% hydrochloride pickling bath as a bright dip to remove the readily soluble lower oxide scale formed thereon in the salt spray oven, and is finally dried in air as clean, bright, stainless-steel metal.

Now having described the process and apparatus hereof, reference should be had to the claims which follows.

I claim:

1. Process for descaling hot sealed metal strip comprisirg continuously passing said hot scaled metal strip into, through and out of a confined molten salt spray descaling zone filled with hot inert gas under pressure, said zone having metal strip entrance and outward openings sealed by evolution of the inert gas under pressure, thereby inhibiting entrance of outside air, maintaining the surfaces of said strip out of contact with handling equipment while the hot strip is being cleaned in said zone, continuously spraying said strip With a hot molten descalng salt atomized and propelled as a spray by a hot inert carrier gas against the surface of said metal strip, said hot carrier gas being applied at a substantially raised pressure sufi'icient to propel said molten salt against the metallic surface as a spray and to supply a positive air inhibiting pressure within said zone around the strip as it enters and as it leaves said zone.

2. The method defined in claim 1 wheren the hot carrier gas is steam.

3. The method as defined in claim 1 wheren the hot carrier gas is heated to a sufliciently high temperature to melt the cleaning salt.

4. The method as defined in claim 1 wheren the hot carrier gas is steam, and the hot metal strip continuously passing through said molten salt spray and having molten salt adhered thereto in the cleaning has its salt removed in a subsequent wash and is quenched to a relatively low temperature with steam at a lower quenching temperature applied continuously to said moving metal strip before the metal leaves said cleaning zone.

References Cted in the file of this patent UNITED STATES PATENTS 1,930,601 Townsend Oct. 17, 1933 2,159,297 Shover May 23, 1939 2,228,836 MacQuaid Jan. 14, 1941 2,320,329 Meduna May 25, 1943 2,447,664 Pegg Aug. 24, 1948 2,458,661 Webster et al. Jan. 11, 1949 2,505,530 Davis Apr. 25, 1950 2,511,797 Hechberg June 13, 1950 2,512,743 Hansell June 27, 1950 2,536,208 Nystrom Jan. 2, 1951 2,635,062 Dunlevy et al. Apr. 14, 1953 2,674,550 Dunlevy et al. Apr. 6, 1954 2,717,845 Charter Sept. 13, 1955 2,762,331 Henderson Sept. 11, 1956 3,004,879 Whitby Oct. 17, 1961 3,022,203 Mains et al. Feb. 20, 1962 

1. PROCESS FOR DESCALING HOT SCALED METAL STRIP COMPRISING CONTINUOUSLY PASSING SAID HOT SCALED METAL STRIP INTO, THROUGH AND OUT OF A CONFINED MOLTEN SALT SPRAY DESCALING ZONE FILLED WITH HOT INERT GAS UNDER PRESSURE, SAID ZONE HAVING METAL STRIP ENTRANCE AND OUTWARD OPENINGS SEALED BY EVOLUTION OF THE INERT GAS UNDER PRESSURE, THEREBY INHIBITING ENTRANCE OF OUTSIDE AIR, MAINTAINING THE SURFACES OF SAID STRIP OUT OF CONTACT WITH HANDLING EQUIPMENT WHILE THE HOT STRIP IS BEING CLEANED IN SAID ZONE, CONTINUOUSLY SPRAYING SAID STRIP WITH A HOT MOLTEN 